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The term 'back up' is a phrasal (or compound) verb. The root word 'back' determines the ending and modification of the verb. So it should be 'backing up' and not 'backuping' or any other such silliness. No more than one would write "He was growuping into a fine young man." or "I am lookuping his phone number". Strictly speaking it's two words but is commonly written as one; probably due to the noun 'backup' being just one word. To avoid this situation one could say "create a backup" but then creating verbs as we need them is evidence of a living language and a technique we share with our French cousins from whom we acquired much of our language.